Abstract [en]

In a nuclear reactor the containment protects the environment from radioactive emissions. It is separated in two parts, a drywell and a wetwell. The drywell contains the reactor vessel and the steam pipes. If a steam leakage occurs in this part of the containment the pressure will increase. To reduce the pressure raising the reactor safety system uses the pressure suppression principle. By this principle the steam is condensed in a condensation pool filled with water. This pool is located in the wetwell part of the containment. To connect the drywell and wetwell there are pipes connected between them. These pipes are submerged under the water level in the condensation pool. This means that steam will be blown down into the condensation pool if the pressure in the drywell exceeds the hydrostatic pressure at the pipe outlet in the pool.

In this thesis the blow down process has been studied with computational fluid dynamics (CFD). The aim was to study the pressure distribution in the condensation pool during a blow down sequence. It is useful to know the pressure distribution and the size of the pressure oscillations to calculate the forces acting on components in the pool and the pool structure. The CFD method has also been compared to the current used method for this type of calculations. The current method at Westinghouse uses a program called SPIEGEL.

Simulations have been made on a model of a blow down pipe and a condensation pool. This model is called the PPOOLEX facility and is set up at Lappeenranta University of Technology in Finland. The facility has a simplified design compared to a real system. The blow down pipe in the PPOOLEX facility has only opening in the bottom of the pipe while a real blow down pipe has several small holes scattered along the pipe wrap. One of the simulations made in this project was based on an experiment performed on the PPOOLEX facility. The results from the simulation were compared to the experimental results. In that experiment only air was blown in to the facility. Another simulation was made where steam was blown into the facility. Two different CFD solvers, CFX and STAR-CCM+, were used to perform the simulations. Because of the great need for computing power, however, mostly STAR-CCM+ was used.

The result of the air simulation showed similar values ​​of the pressure distribution as in experiment. During the simulation also temperatures were measured and compared to the experiment temperature measurements. In these results there were larger differences between the simulation and the experiment, especially for points located in air. Graphic illustrations of volume fractions and pressures in the facility were produced during the simulation.

When steam was used as a source fluid in the simulation some problems appeared in the solution. The CFD software had problems to get a converging solution, probably due to incomplete or incorrect settings for the solver.

The conclusions of this thesis are that CFD can be used to simulate an air blow down process through the process and get reliable information about the pressure distribution in the pool. The method needs to be more developed in steam simulations to be able to calculate the blow down process in a real reactor. CFD is a good tool to produce graphics of the blow down process. Different types of physical properties, such as velocities, volume fractions, and pressure distributions can be illustrated.